SU‐E‐T‐85: Total Body Irradiation (TBI) Optically Stimulated Luminescence In Vivo Dosimetry

Purpose: To adequately monitor the doses for Total Body Irradiation (TBI) patients using opposed fields, we carry out both active and passive in vivo dosimetry, with the passive dosimeter being Optically Stimulated Luminescence (OSL) detectors. The OSLs are embedded in hemispherical brass buildup caps with the OSL/flat surface placed against patient skin. The in vivo dosimetry devices are not repositioned when the patient is rotated, so both entrance and exit dosimetry are carried out, which arguably gives a better midline dose estimate, and improves marrow graft acceptance by minimizing treatment time. The OSLs were calibrated using a 100cm SAD entrance method with brass buildup caps. The buildup caps have a maximum density thickness of 6.6g/cm2; dmax for our 18MV beam is 3.4g/cm2. This method results in an overestimation of exit dose due to the density thickness differences, requiring a correction factor for exit measured doses. Methods: To determine the exit dosimetry correction factor, OSLs were placed on the entrance and exit sides of solid water phantoms under TBI conditions having thicknesses of 10, 30, and 49.2cm. Combinations of OSLs included entrance‐only, exit‐only and combined entrance and exit measured doses. The entrance and exit dose to the dosimeter position was calculated. Results: The exit dose correction factor calculated from the ratio of calculated to measured values was larger than anticipated (∼17%). The correction factor was applied to the exit dose calculations and compared to the combined measurement for verification. Results were within 1.2% (Table 1).The method was tested on ten patients, giving agreement within 1.1±4% (Table 2). Acceptable TBI tolerance is ±10%. Conclusion: While this is acceptable, we will consider using non‐brass buildup caps to obtain smaller correction factors.